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Old 21st February 2012, 01:25 AM   #1
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Default Designing a "hearing assistant". Questions

Well I doubt I'd ever sell it, but I thought it would make a decent project possibly for those of us with family members or friends with bad hearing...or perhaps yourself.

So I'm trying to design a cheap "hearing assistant" from the ground up. I have a few questions.

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Things you should know:

I plan on using two electret condensor microphones. (Standard dime sized ones). These will be unidirectional. 680Ohm impedance I believe.

I'm getting quite good at using Polymorph, (kneadable prototyping thermoplastic), and as funny as this sounds I may design my own headphones from this plastic and put a headphone element inside. There are several on Mouser and Digikey.

I'll embed the mics on each side, just as if they were real ears. I do not plan on putting the electronics in the headphone for I think it would weigh too much. The idea is to have an altoids tin or equivalently sized metal enclosure that would house all of the electronics and would have the volume control and 2-band equalizer I was hoping for.

If you already think I'm crazy, you're probably right, but bear with me.

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Here's what I'm thinking for design:

Mic Preamp- 2x INA217 instrument preamp (BurrBrown/TI)
Equalizer- 2x Passive Baxandall
Headphone Amp- OPA134 or equivalent [cmoy-type] (BurrBrown/TI)
Batteries- Li-On or NiMH cell(s) ~ 5-8V (If NiMH, LM393 comparator used for charging, If Li-Ion...IDK)
Power Supply- NCP3063 step up converter to ~12-14V --> TLE2426 rail splitter to +/- 6-7VDC


Does this sound feasible? I probably would have to do a prototype on a much larger board than an altoids tin, but perhaps after doing some good layout, etching, or getting pcbs made, it can be made into a small device.

Also hoping that a good ground plane will help. Perhaps I will manhattan style prototype it.

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Questions.

For the INA217, the diagram looks like it wants a balanced input, however, I'm not using a balanced setup, just 2 simple electret mics. If I'm unbalanced, on which pins do I input? Anything else changed?

www.ti.com/lit/ds/symlink/ina217.pdf

I would like to have a simple 2 band passive Baxandall equalizer so that the user can reduce bass or treble depending on the environment or anything else.

After what stage should this equalizer go? After the mic preamp or after the headphone preamp...I doubt it would go on the low voltage mic signal...don't want to induce any more noise.

How should I run the various connections to/from the headphone (which also houses the mics)?

My idea was to use a mini-XLR4 type connector and then Mogami quad conductor wire + shield. So one wire for each mic and each headphone element and shared ground. Is that a bad idea?

Finally, does anyone have any good ideas for batteries? And charging. Li-ion is preferred but I'm not too sure about charging them..yet.

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Sorry this is REALLY long, but I'm pretty excited for this and I think it might be an interesting project for me (you?) to try since it's practical.

Thanks a bunch.
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Old 21st February 2012, 01:48 AM   #2
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You might want the reassess how much fidelity you really need for a hearing aid versus size/battery life.

INA217 has low noise but its 10mA quiescent current would drain your batteries rather fast, opa134's Iq isn't low either.

If this doesn't have to be hifi, it might be worthwhile to make it run on single supply and cap couple to the headphones. That way you'd also have a high pass filter which may help reduce handling noise?
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Old 21st February 2012, 02:05 AM   #3
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Thank you for the input.

Yeah I as I was typing that...I was thinking: How the hell am I going to fit this all in an altoids tin?

10mA is quite a bit for batteries, you are certainly right. Perhaps a BJT or FET would be better? Probably could achieve close to 1mA Iq depending on how you set it up.

Good call.

So for the coupling cap, I just put that in series with the headphones? What does a single supply op amp do for noise reduction/ HPF? And yeah I suppose something like a OPA2353 would do well.

Thank you.

Last edited by FenderBender11; 21st February 2012 at 02:08 AM.
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Old 21st February 2012, 02:35 AM   #4
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Perhaps a bandstop filter?

Last edited by FenderBender11; 21st February 2012 at 02:45 AM.
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Old 21st February 2012, 08:05 PM   #5
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Hearing aids typically use AGC/ALC, which makes a whole lot of sense... and modern ones tend to have custom EQ as well (not sure whether a Baxandall-style tone control will suffice).

You may want to get a handle on noise levels at electret capsule output. AFAIK these are relatively noisy, as you'd expect with a predominantly capacitive element on a FET input. Also, a capsule suited for either common source or common drain configuration would enable higher maximum level handling if needed. Watch out for sensitivity, too.

I don't think an OPA(2)134 makes a great headphone driver. OPA2229 or similar may be better suited here (I think that was its number - a low Iq, high Iout,max affair...).
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Old 21st February 2012, 09:11 PM   #6
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Quote:
Originally Posted by FenderBender11 View Post
So for the coupling cap, I just put that in series with the headphones? What does a single supply op amp do for noise reduction/ HPF? And yeah I suppose something like a OPA2353 would do well.
Sine this hearing aid is to be worn, physical noise (walking, chewing..etc) would be coupled really well to the electret capsules, these being low frequency noises could certainly use a bit a filtering.

If running off a single Li-ion battery, ac couple the headphone transducers to the negative side of the battery in series with a 100uF electrolytic cap (and if you are really saving space, tie both transducer return leads to the neg rail with 1 cap). This should give a 1st order HPF with corner frequency around 50hz if you are using a standard 32 ohms headphone transducer. Try sourcing high efficiency 16 ohm drivers if you would like to make it even more battery friendly.

As for opamps, OPA2277 might make a good compromise if you are looking for relatively low Iq, and you can possibly use it as the mic preamp gainstage too, depending on how much gain is actually needed.

Last edited by nereis; 21st February 2012 at 09:14 PM.
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Old 21st February 2012, 10:38 PM   #7
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Thank you. I might be changing gears on the project...more of a "spy" type hearing device? Though the same principles would apply.

Not to be selfish but I would like to use it myself, or atleast have fun with it. I don't require a hearing aid. However, if/when I complete it, the design may be of interest to those who might need a hearing aid...if that makes sense.
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